Relax, Girl: Boyfriend's 'Love Hormone' Wards Off Your Rivals

A hormone known as "the love hormone" prompts men in relationships to keep their distance from other women who are attractive, new research suggests.

Oxytocin, which is known to contribute to pair-bonding, encourages men to expand their personal "Don't come near me" bubble when around an attractive woman — but only when those men are in relationships, researchers found. Single men were just as likely to get close to a pretty stranger whether or not they'd been dosed with oxytocin, according to a study published tomorrow (Nov. 14) in the Journal of Neuroscience.

"Previous animal research in prairie voles identified oxytocin as major key for monogamous fidelity in animals," study researcher Rene Hurlemann of the University of Bonn in Germany said in a statement. "Here we provide the first evidence that oxytocin may have a similar role for humans."

Bonding and dividing

Oxytocin helps keep female prairie voles bonded with their partners. In humans, oxytocin is known to facilitate initial bonding between romantic partners as well as between mothers and children. The hormone also has a dark side, or at least a complex one: It can make people less trusting of those they see as outsiders, actually boosting aggression in some situations. [The History of Human Aggression]

Because the hormone's effects are so dependent on social context, Hurlemann and his colleagues wanted to know if men dosed with a jolt of oxytocin would react differently to beautiful women depending on their relationship status. They recruited 86 heterosexual males and randomly assigned them to get a nasal spray of either oxytocin or a placebo with no effect.

Next, the men participated in experiments to gauge their comfort with unfamiliar but lovely ladies. In the first, the men either approached or were approached by a pretty female experimenter. The men had been told to hold their position, or stop her approach, at the distance where they felt comfortable conversing, establishing a personal bubble. In another experiment, involving a joystick, men saw either pleasant images (beautiful women or landscapes) or disturbing ones (mutilated body parts or dirt). When the image was pleasant, they had to pull the joystick toward them. When the image was unpleasant, they had to push the joystick away.

Keeping love alive

In both experiments, researchers found that relationship status mattered. Men dosed with oxytocin who had girlfriends or wives had bigger personal bubbles than other men. Single men and guys who got the placebo generally established themselves at about 20 to 24 inches (50 to 60 centimeters) away from the attractive experimenter. Oxytocin-dosed guys in relationships stayed about 28 to 30 inches (70 to 75 cm) away.

Guys in relationships who got the oxytocin dose were also slower than other men to respond to photographs of beautiful women in the joystick task, the researchers found.

Oxytocin-dosed men reported no less trust in the female experimenter than other guys did. Nor did oxytocin change men's personal bubbles when they were asked to approach a strange man rather than a strange woman. The findings suggest the result of oxytocin in paired-up guys is all about boosting fidelity.

"In monogamous prairie voles, we know that oxytocin plays an important role in the formation of the pair bond," Larry Young, an oxytocin researcher at Emory University who was not involved in the study, said in a statement. "This study suggests that the general role of oxytocin in promoting monogamous behavior is conserved from rodents to man."

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Stephanie Pappas

Stephanie Pappas is a contributing writer for Live Science. She covers the world of human and animal behavior, as well as paleontology and other science topics. Stephanie has a Bachelor of Arts in psychology from the University of South Carolina and a graduate certificate in science communication from the University of California, Santa Cruz. She has ducked under a glacier in Switzerland and poked hot lava with a stick in Hawaii. Stephanie hails from East Tennessee, the global center for salamander diversity. Follow Stephanie on Google+.